Game is inapt - ruined party (9)
I believe the answer is:
paintball
'game' is the definition.
(I've seen this before)
'inapt ruined party' is the wordplay.
'ruined' indicates an anagram.
'party' becomes 'ball' (ball is a kind of party).
'inapt' is an anagram of 'paint'.
'paint'+'ball'='PAINTBALL'
'is' acts as a link.
(Other definitions for paintball that I've seen before include "messy game" , "Missile used in war games" , "war game" , "simulated combat" .)